Surveillance of a plurality of refrigerated containers and determination of an insulation parameter of a refrigerated container

ABSTRACT

A method is disclosed herein of managing a plurality of refrigerated containers (2), the method comprising the step of surveilling the insulation condition of said containers (2) by repeatedly determining an insulation parameter (Uact, Ucur) of each of the plurality of refrigerated containers (2). Furthermore is disclosed a method to determine an insulation parameter (Uact) of a refrigerated container (2), the method comprising at least the steps of—determining a refrigeration effect (QRef) caused by a refrigeration unit refrigerating the container (2), —calculating an actual rate of energy loss of the container (2) due to heat ingress from the ambient surroundings, —determining an actual temperature difference (ΔT) between the interior (8) of the container (2) and the ambient air, and—determining the actual insulation parameter (Uact) of the container (2) from the ratio of said actual rate of energy loss and said actual temperature difference.

Refrigerated containers require energy for operation in cooling and maintaining cooling temperatures. Over time the energy requirements may change due to the age of the container and different operating conditions.

This at least partly due to the heat leakage caused by heat ingress between the interior of the container and the ambient surroundings is changing over time. The heat ingress primarily changes because the insulation of the container will deteriorate as the insulation material degrades. Specifically, the insulation foam becomes a less efficient insulator over time.

The present inventor has realised that the surveillance of an insulation parameter of refrigerated containers may provide a number of advantages in the use of the containers, such as to rate or rank a plurality of containers in dependence of their insulation condition, e.g. their ability to prevent the containers from warming up due to heat ingress.

Several operations can be improved by having an improved insulation characteristic of the refrigerated container:

-   -   Identification that a container may need repair. The poor         insulation condition may be because the outer skin of the         container has been punctured and needs repair     -   Ranking of containers for suitable jobs better insulating         containers can be used for higher value cargo or priority         customers     -   Indication of placement of a refrigerated container in a ship         the poorer insulated containers should be located on the inside         of the ship and not on the outside, more wind exposed positions.     -   Revision of the estimated lifetime of the refrigerated         container. Better performing containers may be kept in service         longer whilst poorer performing containers may be removed from         service earlier.

The inventor has found that such improvement may be achieved by surveilling the insulation condition each of a plurality of refrigerated containers, such as e.g. all refrigerated containers on a vessel, the refrigerated containers in a depot section for containers or all refrigerated containers on the fleet level of a company, the surveillance method comprising the step of repeatedly determining an insulation parameter of each of the containers.

Such surveillance of a larger group of refrigerated containers would provide detailed data about the status of the insulation condition of the containers as a group.

The development in insulation parameter may be an indicator of general ageing and wear of the insulation, or a sudden drop in insulation of a container may indicate an injury to the container. The quality of repair and maintenance of the containers with respect to the insulation condition may also be detected by such surveillance.

This further method may comprise the step of identifying which of the plurality of containers need maintenance based on the determined insulation parameter or the model insulation parameter of the container.

The further method may also include the step of ranking the plurality of containers for suitable use thereof based on the determined insulation parameter or the model insulation parameter of the container.

Furthermore, the further method may include the step of selecting the placement of each of said plurality of containers in a ship based on the determined insulation parameter or the model insulation parameter.

The further method may also include the step of estimating the lifetime of each container based on the determined insulation parameter or the model insulation parameter of the container.

The insulation parameter of each of the plurality of containers may be determined by means of the set point temperature of that container, the ambient temperature of that container and an energy consumption of that container so than an energy balance in a steady state of the container can be determined, the energy consumption of the container being a measure of the refrigeration effect supplied to the container.

In a particular embodiment, the insulation parameter of each of the plurality of containers is determined by means of the method disclosed below as well as in the appended set of claims.

The inventor has found that such improvement may in particular be achieved by determining an insulation parameter U of the refrigerated container according to the disclosed method determined on the basis of an energy balance equation.

The heat ingress may be modelled by an insulation parameter U (W/K) and the temperature difference ΔT (K) between the inside and outside of the container or box.

Heat ingress=Q _(heat_ingress) =ΔT·U

ΔT=T _(ambient) −T _(box)

It should be noted that the amount of heat leaking from a container is dependent on multiple factors such as weather conditions, ambient temperatures, humidity, placement of the box on the ship, wind conditions etc.

The refrigerated container may be a so-called reefer container, i.e. refrigerated containers having an integral refrigeration unit, in particular an intermodal container (shipping container) used in intermodal freight transport that is refrigerated for the transportation of temperature sensitive cargo.

Thus, herein is disclosed a method to determine an insulation parameter of a refrigerated container, the method comprising at least the steps of

-   -   determining a refrigeration effect caused by a refrigeration         unit refrigerating the container,     -   calculating an actual rate of energy loss of the container due         to heat ingress from the ambient surroundings,     -   determining an actual temperature difference between the         interior of the container and the ambient air, and     -   determining the actual insulation parameter of the container         from the ratio of said actual rate of energy loss         Q_(heat_ingress) and said actual temperature difference.

In a basic method of determining the actual insulation parameter, the rate of energy loss is taken to be equal to the refrigeration effect Q_(Ref caused) by a refrigeration unit refrigerating the container. Thus, it may be assumed that

Q _(heat_ingress) =Q _(Ref)

This is more accurate in case the actual insulation parameter is determined when the container is operated in frozen mode at a temperature set point of the interior of the container of ≤−5° C., preferably ≤−15° C. In such case, there is no contribution to the energy balance from e.g. heating elements inside the container for keeping a specific temperature of commodities as fruit, vegetables or the like which also may contribute to the energy balance by respiration heat and condensation of water vapour released from the commodities due to the respiration.

The actual insulation parameter may be determined between defrost cycles of an evaporator of the refrigeration unit and be initiated when the temperature of the interior of the container has been determined to be stable after defrosting of the evaporator, so that the container and the commodities stored therein are at a constant temperature when the actual insulation parameter is determined, so that contribution to the energy balance due to changes in temperature over time of the container and the commodities is reduced or even for practical use can be considered to be eliminated.

A more complete energy balance may include the contribution Q_(evaporator_fans) from the operation of evaporator fan or fans inside the container as well as the contribution Q_(internal_electrical_consumed) from the consumption of other internal electrical equipment in the container

Q _(heat_ingress) =Q _(Ref) −Q _(evaporator_fans) −Q _(internal_electrical_consumed)

The latter contribution may often be of a minor quantity as compared to the magnitude of the other contributions, but is may be an advantage to calculate the consumed power of evaporator fan or fans of an evaporator of the refrigeration unit and apply said consumed power to calculating the actual rate of energy loss through the insulated outer walls of the container. This may in particular be calculated from applying the supply voltage and the supply frequency of the evaporator fan or fans to calculating the consumed power of the evaporator fan or fans provided that a look-up table, a formula or other means are provided to determine the consumption of electrical power from those input.

The actual insulation parameter of the container is the parameter U_(act) determined at a particular instance under a given set of operating conditions and may for that reason vary. In order to determine a more stable and thus reliable measure of the insulation parameter of the container, the method of determining an actual insulation parameter (U_(act)) is repeatedly performed over time and a current insulation parameter (U_(cur)) of the container is found from said determined actual insulation parameters (U_(act)) of the container. In particular, the current insulation parameter (U_(cur)) of the container may be found by calculating a moving average value of said determined actual insulation parameters (U_(act)) of the container.

The disclosed method may further comprise the steps of:

-   -   determining a difference in insulation parameter between the         determined insulation parameter, i.e. the actual or the current         insulation parameter, and a model insulation parameter for the         container, and     -   updating the model insulation parameter for the container using         the determined insulation parameter.

The refrigerated container may be an intermodal container, the definition of which is currently generally determined by two ISO standards, ISO 668:2013 and ISO 1496-1:2013.

The refrigerated container may receive cooling from a refrigeration unit situated external to the container itself, which e.g. delivers the cooling effect to the container by means of a circulating liquid such as brine or a refrigerant to be evaporated in the refrigerated container. The refrigerated container may alternatively comprise an integrated refrigeration unit for refrigeration of the container, such as in a so-called reefer container.

The refrigeration effect Q_(Ref) released by the refrigeration unit may be determined from the current rotational speed and the current intermittence time of a compressor of the refrigeration unit, provided that a look-up table, a formula or other means are provided to determine the refrigeration effect of the refrigeration unit from those input.

Alternatively or additionally, the refrigeration effect Q_(Ref) released by the refrigeration unit is determined using at least one of the suction pressure at the inlet of the compressor, and the discharge pressure from the compressor, provided that a look-up table, a formula or other means are provided to determine the refrigeration effect of the refrigeration unit from those input.

Thus, the disclosed method may further comprise the step of identifying that the container needs to be maintained based on the determined insulation parameter (U_(act), U_(cur)) or the model insulation parameter (U_(model)) of the container.

Additionally, the disclosed method may further include the step of ranking the container for suitable use thereof based on the determined insulation parameter (U_(act), U_(cur)) or the model insulation parameter (U_(model)) of the container.

Also the disclosed method may further include the step of determining of the placement of a container in a ship based on the determined insulation parameter (U_(act), U_(cur)) or the model insulation parameter (U_(model)).

Yet, the disclosed method may further include the step of estimating the lifetime of a container based on the determined insulation parameter (U_(act), U_(cur)) or the model insulation parameter (U_(model)) of the container.

In another aspect, the present disclosure relates to a method of estimating a respiration rate of chilled, respiring commodities stored in a refrigerated container having insulated outer walls, the method comprising the steps of

-   -   determining an insulation parameter (U_(act), U_(cur),         U_(model)) of the insulated outer walls of the container by         means of the method disclosed herein to determine an insulation         parameter of a refrigerated container,     -   determining a refrigeration effect released by a refrigeration         unit refrigerating the container,     -   determining an actual temperature difference between the         interior of the container and the ambient air,     -   calculating an actual rate of energy loss through the insulated         outer walls of the container from said insulation parameter         (U_(act), U_(cur), U_(model)) and said actual temperature         difference, and     -   estimating the respiration rate from said determined         refrigeration effect and the calculated actual rate of energy         loss.

A number of chilled commodities, such as fresh fruits, vegetables, bulbs, live plants and cut flowers, respires, i.e. convert starch to glucose or glucose to water, heat and CO₂. The key to prolong shelf life of the chilled commodities is to lower their respiration rate, which is achieved by keeping the lowest possible temperature, typically between −1° C., to 20° C., depending on the commodity, and to control the atmosphere inside the refrigerated container, so as to maintain a low content of 02 combined with the highest allowable content of CO₂, the latter depending on the type of commodity in the container. Other components of the atmosphere may also be controlled, in particular the contents of ethylene.

With a reliable estimate of the respiration rate of the chilled, respiring commodities stored in a refrigerated container, the current condition of the commodity is indicated and in case the conditions appear to be deviating from the optimal or requested conditions, action may be taken. Also, the production of CO₂ can be found from the respiration heat and the level of the ventilation rate of the container can be adjusted accordingly in order to reach an allowable content of CO₂ in the atmosphere inside the container so that excessive ventilation of the container is avoided, which requires energy-consuming cooling and often drying of the external air, and so that excessive levels of CO₂ in the atmosphere inside the container are avoided which may lead to so-called CO₂ injuries in the commodities. The level of CO₂ found from determining the respiration heat may take the place of measurements by means of a CO₂ sensor in the storage compartment of the container, in which case the CO₂ sensor is superfluous, or may be used for obtaining a more reliable measure of the CO₂ level inside the storage compartment of the container.

The chemical formula for respiration and the correlation between production of CO₂ and respirational heat is given as:

$\left. {{C_{6}H_{12}O\; 6(s)} + {6{O_{2}(g)}}}\leftrightarrow{{{6{{CO}_{2}(g)}} + {6H_{2}{O(g)}} + {\Delta{G\left( \frac{2872\mspace{14mu}{kJ}}{mol} \right)}}} \approx {1\mspace{14mu}{mol}\mspace{14mu}{{CO}_{2}(g)}}} \right. = {478\mspace{14mu}{kJ}}$

The volume of the produced CO₂ may be found from the ideal gas equation,

$V = \frac{n*R*T}{P}$

which at a temperature of 14° C. and an air pressure of 1 bar is given as

$V = {\frac{1\mspace{14mu}{mol}*8.31\frac{J}{{mol}*K}*\left( {271{^\circ}\mspace{14mu}{C.{+ 14}}{^\circ}\mspace{14mu}{C.}} \right)}{10{0.0}00*{Pa}} = {{0.0}237*{{{{nm}3}\left( {{14{^\circ}\mspace{14mu}{C.}},{1\mspace{14mu}{Bar}}} \right)}/{mol}}}}$

In the method the step of estimating the respiration rate may further comprise one or more of the steps of

-   -   determining the consumed power of the evaporator fan or fans of         an evaporator of a refrigeration unit of the container,     -   determining the consumed power of heating elements of the         container,     -   determining the consumed power of electrical consumers internal         to the container,     -   determining the heat rate released from condensation of water         vapour inside the container, and     -   determining the heat rate inflow into the container due to         ventilation of ambient air into the container.

By including these powers and heat rates, an improved estimate of the respiration rate of the chilled commodities may be obtained.

In a particular example, the supply voltage and the supply frequency of the evaporator fan or fans may be applied to calculating the consumed power of the evaporator fan or fans as discussed previously

Also, the refrigeration effect released by the refrigeration unit may be determined from the current rotational speed and the current intermittence time of a compressor of the refrigeration unit as discussed previously.

The refrigeration effect released by a refrigeration unit may be determined using at least one of the following parameters:

-   -   the suction pressure at the inlet of the compressor, and     -   the discharge pressure from the compressor, which was also         discussed previously.

The respiration rate is mostly estimated when the container is operated at a temperature set point of the interior of the container in the range of −1° C., to 20° C., where most chilled commodities are contained, depending on the particular commodity.

Herein is also disclosed a refrigerated container having insulated outer walls and a data processing device comprising means for carrying out the method disclosed herein.

Furthermore, a computer program product is disclosed comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method as described herein.

Examples of how the present disclosure may be carried out are illustrated in the enclosed drawing of which

FIG. 1 is a perspective cutaway view of a part of an insulated wall of a refrigerated container,

FIG. 2 is a cross-section of a refrigerated container, and

FIG. 3 is a flow chart illustrating the process of determining the insulation parameter of the container and employing it for various purposes.

The insulated wall 1 of a refrigerated container 2 may typically comprise the layers shown in FIG. 1, where on the outside of the container 2 a corrugated steel sheet 3 provides the outermost layer. On the inside, an inner layer 4 is provided made from e.g. aluminium sheets or glass fibre reinforced polymer sheets. Optionally, a plywood layer 5 may be provided under the inner layer 4. Between the outer layer 3 and the inner layer 4, an insulating material 6 such as insulating foam of polyurethane and vertical U-shaped steel beams 7 connecting the inner 4 and outer layers 3 of the container.

In the cross-section of a refrigerated container 2 shown in FIG. 2, the temperature T_(box) inside the container and T_(ambient) are indicated. At the storage space 8 inside the container 2, the commodities to be refrigerated are to be kept. The atmosphere in the storage space 8 is cooled by means of the evaporator 12 delivering the refrigeration effect Q_(Ref) by evaporation of the liquid refrigerant received from the compressor 13. The evaporator fans 9 drive a flow of air from the storage space 8 of the container 2 and past the evaporator 12 in order to cool the air, which is returned to the storage space 8. The amount of water condensing at the evaporator 12 is determined by the condensation sensor 11.

Air exchange between the surroundings of the container 2 and the storage space 8 inside the container for the purpose of controlling the content of the atmosphere inside the container in the storage space 8, in particular the CO₂ contents, is controlled by means of the fresh air ventilators 10.

A controller 14 is arranged to control the operation of the various parts of the equipment in the refrigerated container 2.

An example of how the methods disclosed herein is provided below with reference to the flow chart in FIG. 3. The insulation parameter U of the standard refrigerated intermodal container is from the manufacturing of the new container known to have a value of 43 W·K±1. When the container is operated for storing of commodities that have a temperature set point for storage of e.g. −18° C. or less, the actual insulation parameter U_(act) is determined in step 15. At such low temperatures, the stored commodities do not generate heat from respiration and the fresh air ventilators 10 are not operated. The actual insulation parameter U_(act) is determined between defrost cycles at a time where the temperature T_(box) inside the storage space 8 is stable.

The refrigeration effect Q_(Ref) of the evaporator 12 may be calculated from the mass flow of refrigerant multiplied by the difference between the specific enthalpy of the refrigerant before it reaches the evaporator and the specific enthalpy of the refrigerant after leaving the evaporator.

The electric effect Q_(evaporator_fans) consumed by the evaporator fans 9 and the electrical effect Q_(internal_electrical_consumed) consumed by other minor equipment in the container 2 are determined in order to be able to determine the heat balance for the container and thereby the rate of heat inflow Q_(heat_ingress) into the container 2:

Q _(heat_ingress) =Q _(Ref) −Q _(evaporator_fans) −Q _(internal_electrical_consumed)

With the measurement of the temperature T_(box) inside the storage space 8 and of the ambient temperature T_(ambient), the temperature difference can be determined

ΔT=T _(ambient) −T _(box)

The actual insulation parameter U_(act) for the container can now be determined from the following:

U _(act) =Q _(heat_ingress) /ΔT

However, since the specific circumstances for determining the actual insulation parameter for the container, are subject to variations, it is advantageous to determine the actual insulation parameter U_(act) for the container repeatedly over time, i.e. over weeks or months, and based on those values determine a current value for the insulation parameter U_(cur) for the container from a moving average value of the determined actual insulation parameters in step 16. A model value of the insulation parameter for the container 2 is stored in the controller 14 of the container 2, starting with the factory standard of 43 W·K from new and may be updated when a current insulation parameter has been determined in step 17.

The updated model insulation parameter may be employed to determine the best use of the container and to decide for repair (step 20) in case the insulation parameter U_(model) exceeds a threshold value of e.g. 65 W·K or even scrapping of the container as discussed previously. Also, the updated model insulation parameter U_(model) may be used to rank the container for use, such as deep freezing of commodities for low values of U_(model) or chilling of commodities at temperatures above 0° C. for containers of higher values of U_(model), see step 22, or for determine the most suitable containers for particular placement in a ship in step 23. Also, the expected lifetime of the container may be reevaluated, see step 21. However, an important use of the updated model insulation parameter U_(model) for the container is to calculate a precise estimate of the respiration heat generated by chilled commodities stored in the refrigerated container, i.e. commodities such as fresh fruits, vegetables, bulbs, live plants and cut flowers, which are stored at temperatures where respiration takes place and heat, CO₂ and water vapour are generated in accordance with the formulas provided above in step 19. The heat balance for the container is calculated with the purpose of determining the respiration heat rate Q_(respiration) and for that, the updated model insulation parameter U_(model) for the container is used together with the temperature difference ΔT to determine the heat ingress Q_(heat_ingress).

Thus, the full heat balance equation for a refrigerated container with chilled commodities with respiration and ventilation is:

Q _(Ref) =Q _(evaporator_fans) +Q _(internal_electrical_consumed) +Q _(heating elements) +Q _(respiration) +Q _(condensation) +Q _(ventilation) +Q _(heat_ingress)

Q_(Ref) may be calculated from the mass flow of the refrigerant as discussed previously, alternatively it may be determined from data for the rotational speed of the compressor, the current intermittence time of the compressor, often combined with the suction pressure at the inlet of the compressor and/or the discharge pressure from the compressor.

Q_(evaporator_fans) is the consumed power of the evaporator fans 9.

Q_(internal_electrical_consumed) is consumed power of inside located electrical consumers for instance gas sensors, power electronics etc.

Q_(heating elements) is the consumed electrical power of heating elements placed inside the container.

Q_(condensation) is found from the mass flow of water vapour condensated inside the container as determined by the condensation sensor 11 and the specific latent heat of water, i.e. the specific enthalpy of vaporization.

Q_(ventilation) is found from the ventilation rate, the temperature difference ΔT and the specific heat capacity for air.

Q_(heat_ingress) may be determined from the temperature difference ΔT and the model insulation parameter U_(model).

Q_(respiration) can then be found, which generally provides information about the present condition of the commodities, and more specifically can be used to determine the rate of generation of CO₂ from the respiration as discussed previously, which may be employed in step 24 to control the ventilation rate of the interior storage space 8 of the container 2. 

What is claimed is:
 1. A method of managing a plurality of refrigerated containers, the method comprising the step of surveilling the insulation condition of said containers by repeatedly determining an insulation parameter of each of the plurality of refrigerated containers, wherein the insulation parameter of each of the plurality of containers is determined by means of the set point temperature of that container, the ambient temperature of that container and an energy consumption of that container.
 2. The method according to claim 1, further comprising the step of ranking the plurality of containers for suitable use thereof based on the determined insulation parameter or a model insulation parameter of the container.
 3. The method according to claim 1, further comprising the step of identifying which of the plurality of containers need maintenance based on the determined insulation parameter or a model insulation parameter of the container.
 4. The method according to claim 1, further comprising the step of determining of the placement of each of said plurality of containers in a ship based on the determined insulation parameter or a model insulation parameter.
 5. The method according to claim 1, further comprising the step of estimating the lifetime of each container based on the determined insulation parameter or a model insulation parameter of the container.
 6. (canceled)
 7. (canceled)
 8. A method to determine an insulation parameter of a refrigerated container, the method comprising the steps of determining a refrigeration effect caused by a refrigeration unit refrigerating the container, calculating an actual rate of energy loss of the container due to heat ingress from the ambient surroundings, determining an actual temperature difference between the interior of the container and the ambient air, and determining the actual insulation parameter of the container from the ratio of said actual rate of energy loss and said actual temperature difference.
 9. The method according to claim 8, where the method of determining an actual insulation parameter is repeatedly performed over time and a current insulation parameter of the container is found from said determined actual insulation parameters of the container.
 10. (canceled)
 11. The method according to claim 8, further comprising the steps of: determining a difference in insulation parameter between the determined insulation parameter and a model insulation parameter for the container, and updating the model insulation parameter for the container using the determined insulation parameter.
 12. (canceled)
 13. (canceled)
 14. The method according to claim 8, wherein the refrigeration effect released by the refrigeration unit is determined from a current rotational speed and a current intermittence time of a compressor of the refrigeration unit.
 15. The method according to claim 8, wherein the refrigeration effect released by the refrigeration unit is determined using at least one of the following parameters: a suction pressure at the inlet of the compressor, and a discharge pressure from the compressor.
 16. The method according to claim 8, wherein a consumed power of evaporator fan or fans of an evaporator of the refrigeration unit is calculated and said consumed power is applied to calculating the actual rate of energy loss through the insulated outer walls of the container.
 17. The method according to claim 16, wherein a supply voltage and a supply frequency of the evaporator fan or fans are applied to calculating the consumed power of the evaporator fan or fans.
 18. The method according to claim 8, wherein the actual insulation parameter is determined when the container is operated in frozen mode at a temperature set point of the interior of the container of ≤−5° C.
 19. The method according to claim 8, wherein the actual insulation parameter is determined between defrost cycles of an evaporator of the refrigeration unit and is initiated when the temperature of the interior of the container has been determined to be stable after defrosting of the evaporator.
 20. The method according to claim 8, further comprising the step of identifying that the container needs maintenance based on the determined insulation parameter the model insulation parameter of the container.
 21. (canceled)
 22. (canceled)
 23. The method according to claim 8, further comprising the step of estimating the lifetime of a container based on the determined insulation parameter or the model insulation parameter of the container.
 24. A method of estimating a respiration rate of chilled, respiring commodities stored in a refrigerated container having insulated outer walls, the method comprising the steps of: determining an insulation parameter of the insulated outer walls of the container by means of the method according to claim 8, determining a refrigeration effect released by a refrigeration unit refrigerating the container, determining an actual temperature difference between the interior the container and the ambient air, calculating an actual rate of energy loss through the insulated outer walls of the container from said insulation parameter and said actual temperature difference, and estimating the respiration rate from said determined refrigeration effect and the calculated actual rate of energy loss.
 25. The method according to claim 24, wherein said commodities comprise one or more of: fresh fruits, vegetables, bulbs, live plants and cut flowers. 26-29. (canceled)
 30. The method according to claim 24, wherein the respiration rate is estimated when the container is operated at a temperature set point of the interior of the container in the range of −1° C., to 20° C.
 31. (canceled)
 32. (canceled) 